BOPs have long been used in hydrocarbon recovery operations for preventing well blowouts. A BOP body has a central passageway therein for receiving an oilfield tubular. Radially opposing ram assemblies each mounted on a door are simultaneously actuated by hydraulic pressure to drive corresponding ram blocks into sealing engagement with the oilfield tubular during a well blowout condition, thereby reliably sealing the annulus between the BOP body and the oilfield tubular and preventing a hazardous well blowout condition.
Each BOP ram assembly is supported on a door which sealingly mates with a planar surface of the BOP body. The BOP door is secured to the body by a plurality of large bolts circumferentially spaced about the ram axis. During BOP service operations, the bolts are removed and the door may be structurally disconnected from the BOP body, thereby exposing the ram block for repair or replacement. BOP doors may be mounted on the BOP body by a hinge, as disclosed in U.S. Pat. Nos. 4,253,638 and 5,255,890, so that the BOP body supports the opened door during the service operation. Alternatively, the disconnected BOP doors may each be supported on a pair of rails extending from the BOP body.
Very high well blowout pressures may exist in the central passageway in the BOP body, and the doors must be sealed to the BOP body to prevent leakage of wellbore pressure. Some prior art BOPs rely upon the mechanical pressure applied by the door securing bolts to exert the force required to compress a seal between the door and the BOP body. High wellbore pressure exerts a tensile force on the door securing bolts since the door is forced by the wellbore pressure radially away from the BOP body, thereby creating a slight gap between the door and the BOP body which reduces the effectiveness of the seal. Other BOPs employ a fluid pressure responsive seal, as disclosed in U.S. Pat. Nos. 3,156,475, 4,566,372 and 4,787,654, so that the sealing force is directly related to the wellbore fluid pressure. When using this type of seal between the door and the BOP body, the sealing force thus increases with increased wellbore pressure. When using a pressure responsive seal, tensile force on the bolts still result in a slight gap created between the door and the BOP body when the BOP is subjected to high wellbore pressure, although this gap does not adversely affect the sealing effectiveness since the seal stays engaged with the body as the door moves, i.e., when the door bows, the seal stays flat on the BOP body. The gap between the door and BOP body must be regulated, however, and most importantly the securing bolts must be sized for ensuring that the door does not structurally separate from the BOP body during a high pressure blowout condition.
Those skilled in BOP operations recognize that regular service of the BOP is required to ensure its high reliability. In recent years, BOPs are increasingly used to seal high wellbore fluid pressures, and wellbore fluids are more commonly caustic, contain high water content, or are otherwise deleterious to the life of the ram block seals. Accordingly, the BOP door frequently must be separated from the BOP body to expose and service the ram block seals. The operation of separating the door from the BOP body is no easy task, and frequently requires special tooling sized to accommodate the large diameter bolts. After servicing the ram block seals, the door is reattached to the BOP body by rethreading each of the securing bolts mounted about the periphery of the ram axis. Special torque wrenches are required to ensure that the required torque is applied to each of the circumferentially spaced bolts. The operation of removing a door from the BOP body and reattaching the door after the ram block service operation may require several eight-hour shifts by skilled technicians. Those skilled in hydrocarbon drilling operations recognize that labor costs involved in servicing the BOP ram block seals is nominal compared to the effective cost of the rig down-time during this service operation, since millions of dollars of drilling related equipment may be idle during the BOP service operation.
Another problem with BOPs relates to the increasing axial height required by improved BOPs. Various BOPs have been designed which have advantages over prior art BOPs, but the improved design requires a larger axial spacing between the end faces or flanges of the BOP which each mate with conventional equipment used on a well drilling site. In many applications, the increased axial height required for an improved BOP is not possible or is highly undesirable in view of other parameters involved in equipment effecting the drilling operation. Accordingly, BOPs which provide improvements over the prior art but which result in increased axial length of the BOP body are frequently not widely received in the hydrocarbon recovery industry.
The disadvantages of the prior art are overcome by the present invention, and an improved blowout preventer for use in the hydrocarbon recovery operations is hereinafter disclosed. The blowout preventer of the present invention does not increase the axial height of the BOP, and instead desirably decreases the BOP axial height compared to prior art BOPs. Most importantly, the BOP provides a mechanism for easily and quickly disconnecting the BOP doors from the body during ram block service operations, while still providing a reliable seal between the BOP body and the doors during well blowout conditions.